This invention relates to a method of forming or shaping a bearing bushing used in various kinds of machines, such as an automobile, an industrial machine and an agricultural machine, and more particularly to a bushing (whose outer peripheral surface serves as a sliding surface) used in that portion of such a machine which is liable to receive an impact load and an uneven load.
A bushing whose outer peripheral surface is used as a sliding surface is already known. There are known methods of providing a crowning shape to an outer peripheral surface of a bushing so that the outer diameter of the bushing is smaller at its opposite longitudinal ends, and increases progressively toward the longitudinal central zone of the bushing. In one such known method, those portions of an outer peripheral surface of a bushing disposed adjacent to opposite ends thereof are machined to impart a crowning shape to the outer peripheral surface.
For example, a bushing, mounted as a bearing member in a shock absorber provided at a front fork of a motorcycle, receives an impact at the time of start of the motorcycle, and also is greatly influenced by undulations on the road during running of the motorcycle. Namely, due to vibrations transmitted via wheels at the time of the start and during the running, as well as bending of an inner tube (or a shaft) supporting the bushing, and bending of an outer tube slidable relative to the outer peripheral surface of the bushing, an impact load and an uneven load act on the outer peripheral surface (sliding surface) of the bushing at the ends of the bushing. Therefore, it is necessary that this sliding surface of the bushing should be formed into a crowning shape so that the impact load and the uneven load can be absorbed or alleviated.
In the method of providing the crowning shape to the outer peripheral surface of the bushing by machining the time required for the machining is relatively long, and therefore an enhanced productivity can not be expected. And besides, chips produced by machining deposit on the sliding outer peripheral surface of the bushing, and may damage the sliding inner peripheral surface of the outer tube held in sliding contact with the outer sliding surface of the bushing.